The invention is directed to a wavelength converter which makes use of modulational instability (MI) in a dispersion shifted waveguide fiber to convert the wavelength of an optical signal traveling in a waveguide. In particular, the novel converter makes use of highly non-linear waveguide fiber to achieve wavelength conversion over a wide bandwidth, at high efficiency, using reduced pump power.
Modulational instability (MI) is self phase matched parametric amplification in which the non-linear index of refraction compensates for wave vector mismatch between the pump pulse, and, the signal and converted pulses. That is, the non-linearity of the waveguide, in which the pump pulse and signal pulse overlap, changes the wave vector of the pump pulse to allow pump light energy to transfer into a pulse at the converted wavelength and into the signal pulse. A more thorough explanation of this conversion phenomenon may be found in, "Parametric Amplification and Frequency Conversion in Optical Fibers", R. H. Stolen, J. C. Bjorkholm, Journal of Quantum Electronics, Vol. QE-18, pp. 1062-1072, July, 1982.
The MI in general has a higher conversion efficiency than converters based on four wave mixing. The conversion efficiency of the MI device increases exponentially with length of the waveguide (converter waveguide) in which the signal and pump pulses interact. Also the bandwidth of the MI device can be tuned by changing the pump pulse amplitude, thereby changing the non-linear refractive index.
An additional benefit of the MI device is that it operates in the anomalous dispersion region of the total dispersion vs. wavelength curve. The term anomalous region refers to the portion of the dispersion curve over which shorter wavelength light travel faster in the waveguide than does longer wavelength light. The usual sign convention is dispersion is taken as positive in the anomalous region. In the so called normal region of the dispersion curve, longer wavelength light travels faster and the dispersion is taken to be negative. Thus the pump wavelength is longer than the zero dispersion wavelength of the converter waveguide. Normal manufacturing variations in the converter waveguide fiber will not result in the pump or signal wavelengths falling in the normal dispersion region of the converter waveguide, thereby switching off parametric amplification. Both the converted and the signal pulses may have wavelengths in the anomalous dispersion region so that soliton amplification and switching are made possible. Also, the signal pulse wavelength may be less than or greater than the pump pulse wavelength.
The converter device disclosed in this application may have an all-fiber construction, thus making full use of the mature waveguide fiber technology as well as the associated technologies of connecting, splicing, coupling, filtering and the like.
As demand grows for ever higher data rates incorporated in multi-node architectures, the need for an efficient, wide bandwidth wavelength converter, using readily available components, becomes more urgent.